This invention relates to a unique combination of membrane morphology and hydrophilic pore additive to enhance performance of gas dehydration membranes.
Known methods of enhancing gas dehydration over conventional membrane materials involve various manners of coating the membrane surface, treating the membrane polymer material, or subjecting the membrane to a reaction process to achieve enhanced dehydration performance.
For example, a suitable membrane may be coated with silicon rubber or oil, or with polyvinylpyrrolidone. The result of coating a membrane with these materials is improvement of water-to-gas selectivity while achieving high water permeability. The drawback to this method, however, is that it requires a second treatment step, which is usually done after module manufacture, and may result in a non-functioning module due to the deposition of a membrane coating which is too thick, or a coating which has been deposited incompletely or non-uniformly.
Another means of enhancing membrane performance is related to the actual membrane composition. In U.S. Pat. No. 4,876,009 a composite reverse osmosis membrane of a microporous polymeric support and a polyamide reaction product of a tetrakis-aminomethyl compound and a polyacylhalide is disclosed. The reaction product is an interfacial polymerization (IFP) reaction product, i.e., a surface treatment product or film formed within the porous membrane. Similar membrane composition and construction is disclosed in U.S. Pat. Nos. 4,853,122; 4,781,733; 4,851,127; and 4,772,391. In each instance, the improved separation capability which is reported is achieved by employing a surface treatment technique which deposits a thin film on the surface of a polymeric support material. Drawbacks to this type of construction or membrane generation method are similar to those noted above.
An additional drawback to the foregoing methods is the high gas permeability of the membranes. This can be a particular problem in the dehydration of a mixed gas, such as air, where product composition may be adversely changed due to different permeation rates of the gases being dehydrated.
No method currently used is able to successfully achieve gas dehydration at an acceptable product rate and yet maintain low gas permeability, while eliminating the coating or IFP steps.
What is needed, therefore, is a membrane which achieves gas dehydration at an acceptable product rate while maintaining low gas permeability.
What is further needed is a membrane which achieves gas dehydration at an acceptable product rate while maintaining low gas permeability and which is easily commercially prepared, without having to undergo post- casting coating or reaction processing.
These and other objects of the subject invention will become readily apparent to the skilled artisan upon reading and understanding the full disclosure which follows.